The wastewaters produced from restaurants and other large scale food processing facilities inherently contain large amounts of fats, oils and greases (this is often referred to as FOG and the general waste stream is often referred to as Brown Grease). Wastewater discharges from such facilities must be via a grease trap prior to release to public waste water streams. These grease traps are essentially crude gravity separation devices that prevent carryover of FOG to the public sewer. These traps work reasonably well provided that they are pumped out regularly. This pumping is typically performed using a vacuum truck similar to that used for septic tank pump-out. Disposal of this Brown Grease is difficult and is the subject of this invention. Somewhat similar but less troublesome is the waste grease from deep fat fryer operation. This is usually referred to as “Yellow Grease” and currently has sufficient commercial value that it is not ordinarily disposed of in the grease traps but rather is sold to entities that will remove it from the premises.
Brown Grease typically contains a variety of particulate matter in addition to water and fats, oils, and grease (FOG). Brown Grease accounts for about 300 million gallons of waste per year in the United States and contains from 1 to 5 percent grease. Brown Grease presents a difficult disposal problem in that it is not easily separated into a usable product and if not carefully handled it can upset of normal water treatment processes. The most general utilized disposal method is to reduce the volume by allowing some gravity settling period after which much of the water may be removed. This water may be disposal of my means ordinarily used at a commercial water treatment plant. The concentrated grease trap pumpings (“concentrated” referring to the fact that a preliminary gravity separation has allowed for removal of much of the water originally contained in the pumpings) would be treated by one of the following methods:                1. Adding chemicals to partially solidify the grease into a gel which would be incorporated into a composting operation or simply buried in a landfill, or        2. Biologically degrading the material.        
Some attempts have been made to more completely separate the FOG from the concentrated grease trap pumpings by heating the incoming material and allowing gravity to more completely separate the liquid grease and water—in some cases, augmented by use of centrifugal separators. While further reducing the water content, this approach has not produced an economically useful product because of the high degree of emulsification of the incoming grease trap pumpings. This emulsification is predominately chemical in nature due to the extensive use of detergents in the cleaning process that produced the grease trap wastes and is very resistant to gravity or mechanical separation methods. A classical method for breaking such emulsions would be to perform chemical analysis to determine the best demulsifying chemical and to treat the waste stream accordingly. However, since there is wide variation in cleaning practices that produced the grease trap waste, this is both time consuming and expensive in an operating water treatment facility.
This invention overcomes the problem by reducing the waste grease stream to a concentrated grease/water mixture and then using the relatively high temperature waste heat available in the engine exhaust to heat the mixture and evaporate all the water in the emulsion. The first stage heating in the process serves to liquefy all of the grease contained in the waste grease stream. The centrifugal separation then further concentrates the waste grease stream to the point that sufficient heat is available in the engine exhaust to provide for complete evaporation of the water of emulsion plus any free water in a second heating stage. The resulting fluid (grease fuel) is a water free oil with some fine particulate material that can be readily filtered out prior to use as an engine fuel.
Compression ignition (“diesel”) engines are widely used for production of electric power but are not normally capable of operation with fuels other than diesel fuel (or other hydrocarbon fuels with similar viscosity, cetane number and lubricating behavior) In order to use a non conventional fuel, the fuel must be treated in such a manner that the fuel characteristics are similar to the design basis diesel fuel or the engine fuel ignition system must be modified to allow for the variations. The grease fuel after having been heated and dewatered as described above is sufficiently close to diesel fuel in viscosity, cetane number and lubricity that no additional engine fuel injection system modifications are required.